Chromatography

Question 1

Mobile phase

Answer 1

• the state in which the molecules of the sample can move
• will always be a liquid or a gas
• e.g. liquid solvent, nitrogen gas

Question 2

Stationary phase

Answer 2

• the state in which molecules of the solid do not move
• usually a solid or a liquid supported on a solid
• e.g silica, alumina, oil

Question 3

Retention time

Answer 3

• retention time is the length of the period when the sample is in mobile phase
• depends how soluble the sample is in the mobile phase, and how strongly it is adsorbed into the stationary phase

Question 4

TLC

Answer 4

• Thin Layer Chromatography uses a stationary phase of a thin layer of silica or alumina on a glass plate, and a liquid solvent as mobile phase
• draw a line in pencil near the bottom of the plate, add a drop of sample. Place in a beaker of solvent (level of solvent should be below pencil line)
• solvent moves up the plate carrying mixture with it, at different points parts of the mixture are adsorbed into the stationary phase. When solvent stops moving, mark its height. Use the positions of the spots and the solvent line to calculate Rf

Question 5

Rf values

Answer 5

• Rf = distance travelled by spot/distance travelled by solvent
• unique to certain chemicals, so can be used to identify them from the chromatography plates

Question 6

Gas Chromatography

Answer 6

• used for volatile liquids, mobile phase is inert carrier gas (e.g nitrogen). Stationary phase is viscous oil coating a solid.
• gas is heated and passed through a column containing the stationary phase
• retention time is time between being injected into column and being detected at the other end
• retention time should be different for every component of the mixture

Question 7

Gas Chromatography and Mass Spectrometry

Answer 7

• Gas Chromatography is very good for separating a mixture into its components, each of which can be passed through a mass spectrometer to generate readings that should help to identify the components